When a piece of plywood is bent the material on the concave side of the panel is in compression and the material on the convex side is in tension. So when you splice plywood you what it to be able develop the same tension and compression stresses across the glue joint when the panel is bent at the joint. When the glue surface is perpendicular to the plywood surface, a butt joint (or in this case half the thickness butts), the glue area is too small to develop the tensile strength of the material. The glue fails then bam the remaining half of the wood fails. Gluing the end grain of wood is always weaker than gluing the sides of the wood fibers so when splicing boat panels the surface plys would usually be end grain. The surface fibers (where the wood is stretched or compressed the most) see the most bending stress so it is most important they have the glue surface to transfer their full strength.